Message1338

Attached backtrace from ubuntu 16.04 (firefox 53.0+build6-0ubuntu0.16.04.1, conkeror
1.0.3+git170412+2207-~nightly1) triggered by "click on any link in a mail message in Gmail", for another
example. (Rolling back to firefox-esr worked, confirming at least that the problem is along that upgrade
boundary.)